Construction boards known as dry wall, gypsum wallboard, or plaster board, have been used for many years as a fire barrier in buildings, particularly housing. As is well known, gypsum board comprising a cast gypsum core with cover sheets of paper or the like may be manufactured by mixing calcined gypsum with water to provide a slurry, which is deposited in a layer on a cover sheet, such as multi-ply paper. Another cover sheet is placed on top of the slurry, so that the slurry is sandwiched between the two cover sheets which become the facings of the resultant gypsum board. After the slurry core has set, the board is cut to desired size (usually in thicknesses of 1/2" or 3/8") and dried, as in a kiln. In use, the board is applied to walls, ceilings, etc. by means of clips, nails, or adhesives.
Gypsum wallboard of the type described above has certain fire resistant properties, and as a result building codes which require fire resistant construction assemblies in the interest of public safety normally include gypsum wallboard as a component part, to be installed in various parts of buildings. The important part that wallboard plays in the make-up of fireproof buildings is evidenced by the standards that have been set by governmental agencies, building code authorities, insurance companies, and builders and manufacturers associations for the installation and performance of fire resistant wallboard.
In its role as a fire resistant building component, gypsum wallboard on the walls and ceilings is expected to stay in place for some length of time and deter the spread of fire in a burning building. It appears that, when exposed to the heat and flames of a fire, the paper cover sheets first burn off the core. The cast gypsum core calcines to give up its chemically combined water, and the water is slowly released as steam, effectively retarding heat transmission and disintegration of the board for a time as the gypsum is calcined. However, as the gypsum calcines, it loses its inherent set gypsum strength, and as a result there is a substantial shrinkage of the board at sustained high temperature, with consequent cracking. As it shrinks, it progressively pulls away from the supports to which it is fastened and eventually collapses. This allows the fire to spread and attack adjacent parts of the building.
Standard 3/8" gypsum wallboard possesses dimensional stability and strength to resist the high temperatures normally encountered in a burning building for somewhat less than an hour. However, this provides marginal fire resistance which meets established codes only under limited conditions. In order to effect higher ratings, it has been necessary in the past to use extra-thick wallboard which is not only more costly but also more difficult to erect because of its increased weight. As may be expected, there has been a very substantial effort in the industry to produce boards which are not heavier or thicker but which have been modified to provide higher fire ratings.
Thus, for example, a number of U.S. Pat. Nos. (e.g., 2,526,066; 2,681,863; 2,744,022; 2,853,394; 3,616,173; 4,557,973; and 4,564,544) disclose the addition of fibers such as glass fibers, asbestos and mineral wool fibers to the gypsum slurry in the manufacture of the wallboard. The inclusion of these materials is for the purpose of imparting a mechanical binding effect to hold the calcining gypsum together and prevent it from disintegrating when subjected to the heat of a burning building.
Other U.S. Pat. Nos. (e.g., 2,526,066; 2,744,022; 3,454,456; and 3,616,173) disclose the concept of including unexpanded vermiculite in the gypsum slurry, the concept being that the unexpanded vermiculite will expand when the gypsum core is heated, thus offsetting the undesirable shrinkage of the gypsum component. Other patents, such as U.S. Pat. No. 2,853,394 disclose the concept of including expanded perlite in the gypsum slurry to improve fire resistance and enable production of a dense wallboard having high flexural strength.
The above developments have been valuable contributions, resulting in the production of wallboards having fire ratings in the range between one and two hours. However, in spite of the improvements, the loss of human lives and the destruction of property from fire continues at an unacceptable rate, and there is continued intense effort to find ways of increasing the fire resistance of building materials.
It is an object of the present invention to provide a construction board which possesses significantly enhanced fire suppressing properties.
It is another object of the invention to produce a construction board containing a lightweight component which serves not only to dissipate the heat of a building fire but also to strengthen and prevent fragmentation of the core material when subjected to fire.
It is a further object to improve the fire resistance of construction board and enhance the integrity of the board to the extent that the need for cover or facing sheets is eliminated.
Other objects and advantages will become apparent as the specification proceeds.